1. Field of the Invention
The present invention is directed to a gradient coil system of the type that is fashioned as a hollow cylinder (tube), with the interior cross-section being elliptical.
2. Description of the Prior Art
Magnetic resonance tomography is a known technology for acquiring images of the inside an object, particularly, the body of a living examination subject. To this end, a magnetic resonance tomography apparatus contains a basic field magnet system and a gradient coil system. The basic field magnet system has, for example, a cylindrical hollow opening. The gradient coil system, which, for example, is fashioned in a tubular manner is arranged in this opening. An imaging volume is situated within the hollow opening of the gradient coil system. The basic field magnet system generates an optimally uniform, static basic magnetic field at least within the imaging volume. The basic magnetic field is co-linear with the principal cylinder axis, to which the z-axis of a Cartesian coordinate system is usually allocated. At least within the imaging volume, the gradient coil system superimposes rapidly switched magnetic fields with approximately constant gradients, referred to as gradient fields, on the basic magnetic field. Three gradient fields corresponding to the three spatial directions of the Cartesian coordinate system can be generated. At any arbitrary point in time, the gradient of the gradient field of a specific spatial direction is of approximately the same size independently of its location within the imaging volume. Since the gradient fields are time-variable magnetic fields, the above in fact applies to every point in time, but the magnitude of the gradient is variable from one point in time to another point in time. The direction of the gradient is usually permanently prescribed by the gradient coil design and is co-linear with one of the spatial directions of the Cartesian coordinate system.
For generating the gradient field, corresponding currents are generated in the gradient coil. The amplitudes of the required currents amount to several 100 A. Since the gradient switching times should be as short as possible, the current rise and decay rates amount to several 100 kA/s. For power supply, the gradient coil is connected to a gradient amplifier. Since the gradient coil represents an inductive load, correspondingly high output voltages of the gradient amplifier are required for generating the aforementioned currents.
In many magnetic resonance apparatuses, the hollow opening of the basic field magnet system is circular-cylindrical, and the gradient coil system is fashioned correspondingly circular-cylindrical. The diameter of the opening of the basic field magnet system is a cost-relevant quantity, particularly given super-conducting basic field magnet systems. The basic field magnet system becomes more expensive the larger the diameter must be fashioned in order to accommodate the gradient coil system therein.
U.S. Pat. No. 5,177,441 discloses a tubular gradient coil system with an elliptical cross-section. Compared to a comparable circular-cylindrical gradient coil system, lower inductances can, among other things, be achieved given comparable gradient field properties. As a result, faster switching speeds for the gradient fields and/or gradient amplifiers with lower power can be realized. High switching speeds are thereby important, particularly for sequences referred to as fast pulse sequences, and a gradient amplifier having lower power saves costs.
A disadvantage of the gradient coil system disclosed in U.S. Pat. No. 5,177,441 is that the two transverse gradient coils for generating gradient fields co-linearly with the minor and major ellipse axes differ greatly in terms of their geometry and their technical data. As a result, the two gradient coils particularly exhibit different inductances as well as different ohmic resistances. In order to achieve comparable switching speeds for the two gradient coils, their gradient amplifiers therefore must be designed with different powers and are differently loaded during operation. In the gradient coil systems with an elliptical cross-section according to the aforementioned patent, further, one of the transverse gradient coils is arranged closer on average to the circular-cylinder cladding of the hollow opening of the basic field magnet system than the other. As a result thereof, the disturbing effects produced in the basis field magnet system by the gradient coils via eddy current induction differ for the two gradient coils. Given employment of an active shielding for reducing these disturbing effects (artifacts), the active shielding must be differently designed for the two gradient coils in order to achieve a comparable shielding effect, which is difficult. A different shielding effect must be accepted given a comparably fashioned shielding.
It is an object of the present invention to provide a gradient coil system of the type initially described that alleviates the aforementioned disadvantages of the above-described conventional systems.
This object is inventively achieved in that an elliptical gradient coil system wherein the gradient coils are arranged such that a first gradient and a second gradient have an oblique position relative to the major and the minor ellipse axes. These two transverse gradient coils generate gradient fields whose gradients are not co-linear with one of the ellipse axes, as in conventional systems. More freedom is thus available in the arrangement of the gradient coils within the gradient coil system to match the two gradient coils to one another in terms of their geometrical configuration and their technical data. A xe2x80x9ctransverse gradient coilxe2x80x9d means a gradient coil whose gradient is orthogonal relative to the direction of the basic magnetic field, at least in the imaging volume.
In an embodiment, the first and the second gradient exhibit an identical angle of inclination relative to the minor ellipse axis.
In another advantageous development, the gradient coils are arranged and designed such that the maximally producible first gradient and the maximally producible second gradient are the same in terms of magnitude. Particularly in combination with the immediately preceding embodiment, this allows the two gradient coils to be implemented approximately the same in terms of their geometrical design and their technical data.
In another embodiment, the gradient coils are arranged such that the first and the second gradients exhibit an angle of inclination in a range from approximately 45xc2x0 through 80xc2x0 relative to the minor ellipse axis. Particularly in combination with the aforementioned embodiments, this allows a gradient resulting from the first and second gradients to assume a lower maximum value (in terms of magnitude) in the direction of the minor ellipse axis than the resulting gradient in the direction of the major ellipse axis. This advantageously allows a direction-dependent stimulation sensitivity of a patient to be taken into account. This is true given the condition that the patient, whose body approximately corresponds to a cylinder having an elliptical cross-section, is disposed as concentrically as possible in the hollow opening of the gradient coil system. It is thereby known with respect to the direction-dependent stimulation sensitivity that gradient fields that frontally penetrate the patient, particularly in the sagittal direction, are especially prone to produce, which are an unwanted side effect produced by the gradient fields. German OS 42 52 592 is referenced for a detailed explanation of such direction-dependent stimulation sensitivity.
In a further embodiment, the gradient coils are arranged and designed such that the first and the second gradient coils have an identical inductance. As a result, identical switching speeds can be achieved for both gradient coils, with gradient amplifiers having the same power being advantageously utilized for both gradient coils. Further, it is implicit in the aforementioned arrangement and design of the gradient coils that these advantageously have approximately the same Ohmic resistances and that their dissipated power is thus the same.
In another embodiment, the first and second gradient coils are arranged and fashioned essentially symmetrically to one another relative to a first plane defined by the minor ellipse axis and the principal axis of the hollow opening, as well as relative to a second plane defined by the major ellipse axis and the principal axis of the hollow opening.
In another embodiment, a shielding coil for at least one of the gradient coils is arranged around the gradient coils with a circular-cylindrical shape. Due to this fashioning of the shielding coil, a high shielding effect can be achieved, especially in the region of the apex of the minor ellipse axis, as a result of the large distance between the gradient coil and the shielding coil. Moreover, the same shielding effect is achieved for both gradient coils given gradient coils arranged symmetrically relative to one another and a shielding of both gradient coils with shielding coils described above.
In a further embodiment, a first circuit includes the first gradient coil and a second circuit includes the second gradient coil, and the first and second circuits are connected to one another via a transformer whose mutual inductance, with operational sign inversion, is the same as a mutual inductance between the first and second gradient coils. As a result, a magnetic coupling of the two gradient coils dependent on the design of the gradient coils is compensated and a reactance of the two gradient amplifiers that is unfavorable for control reasons thereby is prevented.
In another embodiment, a coupling factor of the transformer is optimally equal to one. As a result, a increase in the load inductance for the two gradient amplifiers due to the transformer is kept optimally slight, and the maximally possible switching speeds of the gradient fields are not lowered.
In another embodiment, the gradient coils are free of conductor segments in the region of the apex of the major ellipse axis. This allows a smaller outside diameter of the gradient coil system to be achieved compared to a comparable hollow circular-cylindrical gradient coil system with comparable gradient field properties. The hollow opening of the basic field magnet system can, accordingly, be implemented smaller and the basic field magnet system is thus smaller and less expensive.